I'm a VC, and when I read the line "buying sole control" I assumed that the Series A investment from Benchmark is intact, and their shares still vote on any protective provisions granted to the preferred stock, but Adam simply bought enough stock in the Series B (in combination with his originally granted founders shares) to do two things:

1) represent a majority of all outstanding stock (both common and preferred). 2) represent a majority of all preferred stock.

If the preferred stock votes on all protective provisions in an all-for-one manner, then Adam would control the preferred voting, assuming #2 is true above. All-for-one means that all classes of preferred stock vote together as one group, and each share, regardless of class, gives you one vote. If the governance issues that affect the preferred stock are not setup as "all-for-one" voting, then the other likely situation would be independent class voting, in which case Benchmark and Adam would each get their own veto (Benchmark would control the voting of the Srs A and Adam would control the voting of the Srs B, and so they could each vote the other party down). If Adam really has sole control, then preferred voting would need to happen in an "all-for-one" manner.

My explanation is just an assumption based on that one flip line, but that's how it reads to me.